The utilization of transition metals as polymerization catalysts has led to many vast improvements in the industrial synthesis of polymeric materials. Some examples of these improvements are higher rates of monomer consumption, polymer molecular weight control, and stereochemical control over polymer microstructure.
Stereochemical control of polymerization using .pi.-allylic complexes of transition metals was reported by G. Wilke who obtained a polybutadiene with a predominantly cis-1,4 content using (.pi.-allyl).sub.2 Co(I) as the catalyst, and a high 1,2-polybutadiene with a (.pi.-allyl).sub.3 Cr [Angew. Chem. (Intern. Ed.), 2: 105 (1963)]. Porri et al. disclosed polymerization of butadiene by bis-(.pi.-allyl nickel iodide) to crystalline trans-1,4-polymer and by bis-(.pi.-allyl nickel chloride) to a predominantly cis-1,4-polymer [J. of Polymer Science, 16:2525-2537 (1967)]. After these studies were reported, numerous others have investigated the stereospecific polymerization with .pi.-allylic derivatives of transition metals.
For example, U.S. Pat. No. 3,719,653 to Dawans describes a stereospecific polymerization of conjugated diolefins with a catalyst which is the reaction product of allyl halogenacetate and nickel carbonyl or nickel olefin complex. One reaction product, .pi.-allylnickel trifluoroacetate, is disclosed as being a fairly active initiator for the polymerization of 1,3-butadiene.
-allylnickel trifluoracetate has also been shown to be a fairly active initiator for the polymerization of other monomers. For example, Kormer et al. demostrated the polymerization of cycloolefins [J. Poly. Sci. Pt. A-1 10:251-258 (1971)]. Durand et al. showed that the addition of ligands to a .pi.-allylic nickel trifluoroacetate catalyst system can influence the stereospecific course of the polymerization reaction of isoprene [Polym. Lett. 8:743-747 (1970)]. Fayt et al. show block copolymers of butadiene and styrene formed by the polymerization of butadiene using a .pi.-allylic nickel trifluoroacetate catalyst and a chloranil ligand with the subsequent addition of styrene monomer and additional ligand [J. Poly. Sci. Polym. Chem. Ed. 23:337-342 (1985)]. It has been reported that allylnickel compounds have also been promoted to active initiators by the addition of electron-deficient additives such as TiCl.sub.4, AlCl.sub.3, chloranil, and bromanil [Porri et al., supra; Hadjiandreou et al. Macromolecules 17:2455-2456 (1984)]; however, results using chloranil have been unreproducible [Novak et al., unpublished results]. The polymerization of isocyanides using .pi.-allylic nickel trifluoroacetate catalysts was demonstrated by Deming et al. [Macromolecules 24:6043-6045 (1991)]. Additionally, it has been reported that chloral, trichloracetic acid, and hexachloroacetone were added to some allylnickel halides to prepare polybutadiene [Azizov Dokl. Akad. Nauk SSSR 265(2):362 (1982)].
Current goals in polymer syntheses using metal initiators include increasing conversion rates and stereochemical control, as well as the polymerization of monomers containing polar functional groups. While stereochemical control in the polymerization of various monomers using .pi.-allylnickel halogenacetates has been demonstrated, the activity of these systems are generally fairly low. It would be desirable to increase the monomer consumption in these systems, while maintaining or improving stereoselectivity. In addition, it would be desirable to have a system that is capable of polymerizing a variety of monomers.